1. Field of the Invention
The present invention relates to chiral dopants, and in particular relates to chiral dopants used in cholesterol liquid crystals.
2. Description of the Related Art
A cholesterol liquid crystal display is a display using ambient light as a light source, and therefore a backlight module is not required, saving manufacturing costs. A cholesterol liquid crystal display has bistable characteristics, including a planar state and a focal conic state. Advantages of a cholesterol liquid crystal display include low-electricity consumption and high reflection rate, and capability of reflecting various wavelengths, making it particularly suitable for electronic books.
Cholesterol liquid crystals can be classified into two types, including: (1) nematic liquid crystal molecules which are chiral molecules; or (2) a combination of chiral molecules and achiral nematic liquid crystal molecules, which is generally used in modern cholesterol liquid crystal displays, wherein the pitch of the liquid crystal is controlled by the ratio of the chiral molecules to the achiral nematic liquid crystal molecules in the mixture. According to Bragg's reflection law, the relationship between reflection wavelength (λ) and the pitch (P) of the cholesterol liquid crystal layer can be expressed by λ=P×n, wherein n is birefringence of the cholesterol liquid crystals.
The ability of a chiral molecule to twist liquid crystals is defined as helical twisting power (HTP.) The relationship between the helical twist power, pitch, and concentration of the chiral molecules in the nematic liquid crystal composition can be expressed by HTP×c=1/P, wherein c is concentration of chiral molecules in the nematic liquid crystal composition. As shown in above formula, under a constant pitch, the larger the HTP, the lower the required chiral molecule concentration. Since the addition of chiral molecules will increase the viscosity of the composition, resulting in an increase of the driving voltage of the display and a decrease of the reflection rate, it could be advantageous to increase the HTP value of the chiral molecules to reduce the amount of the chiral molecules required, and thus alleviating the problems such as high viscosity, high driving voltage, and low reflection rate.
Liquid crystal compositions can be generally classified into active matrix compositions and passive matrix compositions. In passive matrix liquid crystal displays, the orientation of the liquid crystals are controlled through voltage applied by a passive device such as a capacitor. On the other hand, in active matrix liquid crystal displays, voltage is applied by an active display such as a transistor. One problem with the existing chiral compounds is that a high HTP generally accompanies low solubility (usually less than 3%) in an active liquid crystal composition.